Zinc/air depolarized cells are typically in the form of miniature button cells which have particular utility as batteries for electronic hearing aids including programmable type hearing aids. Such miniature cells typically have a disk-like cylindrical shape of diameter between about 4 and 20 mm, typically between about 4 and 16 mm and a height between about 2 and 9 mm, preferably between about 2 and 6 mm. Zinc air cells can also be produced in somewhat larger sizes having a cylindrical casing of size comparable to conventional AAAA, AAA, AA, C and D size Zn/MnO2 alkaline cells and even larger sizes.
The miniature zinc/air button cell typically comprises an anode casing (anode can), and a cathode casing (cathode can). The anode casing and cathode casing each have a closed end an open end and integral side walls extending from the closed end to the open end. The anode casing is fitted with an insulating seal ring which tightly surrounds the anode casing side wall. Anode material is inserted into the anode casing. Air diffuser, electrolyte barrier material, and cathode assembly are inserted into the cathode casing adjacent air holes in the cathode casing. After the necessary materials are inserted into the anode and cathode casings, the open end of the cathode casing is typically pushed over the open end of the anode casing during assembly so that a portion of the cathode casing side walls covers a portion of the anode casing side wall with insulating seal therebetween. The anode and cathode casing are then interlocked in a second step by crimping the edge of the cathode casing over the insulator seal and anode casing. During the crimping procedure (or in a separate step) radial forces are also applied to the cathode casing walls to assure tight seal between the anode and cathode casings.
The anode casing of zinc/air button cells may be filled with an anode mixture comprising particulate zinc. Typically, the anode mixture contains mercury, which is amalgamated with the zinc particles. The mercury typically comprises between about 2 and 5 percent by weight based on the weight of zinc. The presence of mercury in the anode improves the zinc interparticle conductivity and reduces cell gassing. The anode mixture also contains a small amount of a gelling agent and the zinc particles become gelled when electrolyte is added to the mixture. Alternatively, the anode mixture may be mercury free, that is, containing no added mercury. In such case added care must be taken to seal the cell tightly due to the somewhat increased gassing, which can be expected. The electrolyte is conventionally an aqueous solution of potassium hydroxide. The closed end of the cathode casing (when the casing is held in vertical position with the closed end on top) may have a flat raised portion near its center. This raised portion forms the positive terminal and typically contains a plurality of air holes therethrough. In this design, the cathode casing closed end also typically has an annular recessed step which surrounds the raised positive terminal. Alternatively, the closed end of the cathode casing may be completely flat across its diameter, that is, without any raised portion at its center. In such design the central portion of such flat area at the closed end of the cathode casing typically forms the cell's positive terminal. In either case, the closed end of the cathode casing of button zinc/air cells is punctured with one or more small air holes to allow air to enter the cell. Such air then traverses an air diffusion layer (or air diffuser) in order to reach the cathode disk.
The air diffuser material is normally composed of one or more sheets of air permeable paper or porous cellulosic material. Such permeable paper or porous cellulosic material allows incoming air to pass uniformly to the cathode assembly and also may serve as a blotter to absorb minor amounts of electrolyte which may leak into the air inlet space. The air diffuser is normally placed first within the cathode casing against the closed end of the cathode casing so that it abuts the air holes. The air diffuser material fills such air inlet space and covers the air holes in the closed end of the cathode casing. Commercial button size zinc/air cells which are commonly used in hearing aid devices may have only one air hole or may have a plurality of small air holes, for example, between 2 and 6 air holes and even more depending on cell size.
Catalytic material typically comprising particulate manganese dioxide (or mixture of various manganese oxides), carbon, and hydrophobic binder can be compacted into a disk shape forming a cathode disk within a cathode assembly. The cathode assembly with cathode disk therein can then be inserted into the cathode casing over the air diffuser on the side of the air diffuser that faces away from the air holes. Typically a cathode assembly is formed by laminating a layer of electrolyte barrier material (hydrophobic air permeable film), preferably Teflon (polytetrafluoroethylene), to one side of the catalytic cathode facing the air diffuser and an electrolyte permeable (ion permeable) separator material to the opposite side of the catalytic cathode disk.
The cathode assembly may have a flat or domed shape. The flat cathode assemblies are preferred, since they are easier and more economical to fabricate. Representative zinc/air button cells with flat cathode assemblies are shown in U.S. Pat. No. 5,279,905; U.S. Pat. No. 6,602,629 B1; and U.S. Pat. No. 6,830,847 B2.
If the cell is not adequately sealed, electrolyte can migrate around the catalytic cathode assembly and leak from the cathode casing through the air holes. Also electrolyte leakage can occur between the crimped edge of the cathode can and insulator if this area is not tightly sealed. The wall thickness of commercial zinc/air button cells are typically greater than about 6 mil (0.152 mm), for example, between about 6 and 15 mil (0.152 and 0.381 mm). The potential for leakage is greater when the anode casing and cathode casing is of very thin wall thickness, for example, between about 2 and 5 mil (0.0508 and 0.127 mm). Such low wall thickness is desirable, since it results in greater internal cell volume.
After the cell is assembled a removable tab is placed over the air holes on the surface of the cathode casing. Before use, the tab is removed to expose the air holes allowing air to ingress and activate the cell.
It is desirable to improve performance of the zinc/air cell regardless of whether mercury is added to the anode or whether the cell is mercury free, that is, contains no added mercury.
It is desirable to improve the service life of the zinc/air cell, under normal usage conditions, without promoting cell leakage.